arch/x86/include/asm/refcount.h

   1 #ifndef __ASM_X86_REFCOUNT_H
   2 #define __ASM_X86_REFCOUNT_H
   3 /*
   4  * x86-specific implementation of refcount_t. Based on PAX_REFCOUNT from
   5  * PaX/grsecurity.
   6  */
   7 #include <linux/refcount.h>
   8
   9 /*
  10  * This is the first portion of the refcount error handling, which lives in
  11  * .text.unlikely, and is jumped to from the CPU flag check (in the
  12  * following macros). This saves the refcount value location into CX for
  13  * the exception handler to use (in mm/extable.c), and then triggers the
  14  * central refcount exception. The fixup address for the exception points
  15  * back to the regular execution flow in .text.
  16  */
  17 #define _REFCOUNT_EXCEPTION                             \
  18         ".pushsection .text..refcount\n"                \
  19         "111:\tlea %[counter], %%" _ASM_CX "\n"         \
  20         "112:\t" ASM_UD2 "\n"                           \
  21         ASM_UNREACHABLE                                 \
  22         ".popsection\n"                                 \
  23         "113:\n"                                        \
  24         _ASM_EXTABLE_REFCOUNT(112b, 113b)
  25
  26 /* Trigger refcount exception if refcount result is negative. */
  27 #define REFCOUNT_CHECK_LT_ZERO                          \
  28         "js 111f\n\t"                                   \
  29         _REFCOUNT_EXCEPTION
  30
  31 /* Trigger refcount exception if refcount result is zero or negative. */
  32 #define REFCOUNT_CHECK_LE_ZERO                          \
  33         "jz 111f\n\t"                                   \
  34         REFCOUNT_CHECK_LT_ZERO
  35
  36 /* Trigger refcount exception unconditionally. */
  37 #define REFCOUNT_ERROR                                  \
  38         "jmp 111f\n\t"                                  \
  39         _REFCOUNT_EXCEPTION
  40
  41 static __always_inline void refcount_add(unsigned int i, refcount_t *r)
  42 {
  43         asm volatile(LOCK_PREFIX "addl %1,%0\n\t"
  44                 REFCOUNT_CHECK_LT_ZERO
  45                 : [counter] "+m" (r->refs.counter)
  46                 : "ir" (i)
  47                 : "cc", "cx");
  48 }
  49
  50 static __always_inline void refcount_inc(refcount_t *r)
  51 {
  52         asm volatile(LOCK_PREFIX "incl %0\n\t"
  53                 REFCOUNT_CHECK_LT_ZERO
  54                 : [counter] "+m" (r->refs.counter)
  55                 : : "cc", "cx");
  56 }
  57
  58 static __always_inline void refcount_dec(refcount_t *r)
  59 {
  60         asm volatile(LOCK_PREFIX "decl %0\n\t"
  61                 REFCOUNT_CHECK_LE_ZERO
  62                 : [counter] "+m" (r->refs.counter)
  63                 : : "cc", "cx");
  64 }
  65
  66 static __always_inline __must_check
  67 bool refcount_sub_and_test(unsigned int i, refcount_t *r)
  68 {
  69         GEN_BINARY_SUFFIXED_RMWcc(LOCK_PREFIX "subl", REFCOUNT_CHECK_LT_ZERO,
  70                                   r->refs.counter, "er", i, "%0", e, "cx");
  71 }
  72
  73 static __always_inline __must_check bool refcount_dec_and_test(refcount_t *r)
  74 {
  75         GEN_UNARY_SUFFIXED_RMWcc(LOCK_PREFIX "decl", REFCOUNT_CHECK_LT_ZERO,
  76                                  r->refs.counter, "%0", e, "cx");
  77 }
  78
  79 static __always_inline __must_check
  80 bool refcount_add_not_zero(unsigned int i, refcount_t *r)
  81 {
  82         int c, result;
  83
  84         c = atomic_read(&(r->refs));
  85         do {
  86                 if (unlikely(c == 0))
  87                         return false;
  88
  89                 result = c + i;
  90
  91                 /* Did we try to increment from/to an undesirable state? */
  92                 if (unlikely(c < 0 || c == INT_MAX || result < c)) {
  93                         asm volatile(REFCOUNT_ERROR
  94                                      : : [counter] "m" (r->refs.counter)
  95                                      : "cc", "cx");
  96                         break;
  97                 }
  98
  99         } while (!atomic_try_cmpxchg(&(r->refs), &c, result));
 100
 101         return c != 0;
 102 }
 103
 104 static __always_inline __must_check bool refcount_inc_not_zero(refcount_t *r)
 105 {
 106         return refcount_add_not_zero(1, r);
 107 }
 108
 109 #endif